The Influence of Tropical Cyclone Size on Its Intensification

Abstract

his study investigates tropical cyclones of the past two decades (1990?2010) and the connection, if any, between their size and their ability to subsequently undergo rapid intensification (RI). Three different parameters are chosen to define the size of a tropical cyclone: radius of maximum wind (RMW), the average 34-knot (kt; 1 kt = 0.51 m s?1) radius (AR34), and the radius of the outermost closed isobar (ROCI). The data for this study, coming from the North Atlantic hurricane database second generation (HURDAT2), as well as the extended best-track dataset, are organized into 24-h intervals of either RI or slow intensification/constant intensity periods (non-RI periods). Each interval includes the intensity (maximum sustained surface wind speed), RMW, AR34, and ROCI at the beginning of the period and the change of intensity during the subsequent 24-h period. Results indicate that the ability to undergo RI shows significant sensitivity to initial size. Comparisons between RI and non-RI cyclones confirm that tropical cyclones that undergo RI are more likely to be smaller initially than those that do not. Analyses show that the RMW and AR34 have the strongest negative correlation with the change of intensity. Scatterplots imply there is a general maximum size threshold for RMW and AR34, above which RI is extremely rare. In contrast, the overall size of the tropical cyclones, as measured by ROCI, appears to have little to no relationship with subsequent intensification. The results of this work suggest that intensity forecasts and RI predictions in particular may be aided by the use of the initial size as measured by RMW and AR34.